1. Field of the Invention
This invention relates to plated parts, such as the semiconductor parts, and their production.
2. Description of the Prior Art
Hermetic sealing covers for a container for a semiconductor device, a method for its fabrication and a method for fabricating a sealed container are described in U.S. Pat. Nos. 3,874,549, 3,946,190 and 3,823,468 to Hascoe. Such sealing covers (or lids), ladder frames and other metal parts used in the fabrication of metallized ceramic packages are conventionally electroplated in order to provide a package which is resistant to salt atmosphere corrosion, among other reasons. One specification to which such finished packages are typically required to adhere is Mil-Std-883C, Method 1009.4, Test Condition A, Salt Atmosphere (Corrosion).
The sealing covers described in the aforementioned patents are conventionally formed of a base material which is electroplated. The base material is usually Kovar or Alloy 42, both of which contain iron as a major constituent. The electroplating sequence is to plate the base metal with nickel and then with gold. A method of fabricating and plating such covers are disclosed in U.S. Pat. No. 4,284,481 to Hascoe, and the plated covers are disclosed in U.S. Pat. No. 4,243,729 to Hascoe. The disclosures of all of the aforementioned Hascoe patents are expressly incorporated herein by reference.
The industry experience has been that it is extremely difficult to pass the salt atmosphere corrosion test and the results obtained are not always reproducible.
Electroplating of metal parts such as semiconductor package parts produces very thin coatings. Thicknesses are usually in the one to five hundred microinch range. Such plating thicknesses exhibit porosity which may leave tiny pinhole paths between the surface of the basis metal and the top surface of the electroplated coating. Salt atmosphere which contains ionized sodium and chlorine is very corrosive. It reacts with the iron, and other constituents of the base metal to form oxides. The salt atmosphere penetrates through the tiny pinholes to cause corrosion of the electroplated metal parts and failure to meet the aforementioned specification.
The most commonly used electroplated coatings on semiconductor packages are nickel on the basis metal and gold on the nickel. Nickel acts as a diffusion barrier to keep the gold from diffusing into the basis metal at elevated temperatures such as occur during solder sealing. Nickel also acts as a corrosion barrier. The outside coating of gold has two functions. It maintains an oxide-free surface for the soldering of the package and because of its oxidation resistant characteristic acts as a corrosion barrier. Typically, a layer of 100 microinches of pure nickel is plated on the base layer, and then a layer of 50 microinches of pure gold.
In order to increase the corrosion resistance of plated parts, it would appear that the amount of gold deposited on the substrate should be increased. However, this would greatly increase the cost of the plated part and requires a significant increase in thickness of the gold layer before any appreciable increase in corrosion resistance is noted. It would be desirable to provide plated parts which have improved salt atmosphere corrosion resistance, as well as a process for producing such parts. It would also be desirable to provide parts which have the requisite corrosion resistance, without an economic penalty.